dm cache: add data block size limits to code and Documentation

The origin is divided up into blocks of a fixed size.  This block size

is configurable when you first create the cache.  Typically we've been

using block sizes of 256k - 1024k.

using block sizes of 256KB - 1024KB.  The block size must be between 64

(32KB) and 2097152 (1GB) and a multiple of 64 (32KB).

Having a fixed block size simplifies the target a lot.  But it is

something of a compromise.  For instance, a small part of a block may be

#define MIGRATION_COUNT_WINDOW 10

/*

 * The block size of the device holding cache data must be >= 32KB

 * The block size of the device holding cache data must be

 * between 32KB and 1GB.

 */

#define DATA_DEV_BLOCK_SIZE_MIN_SECTORS (32 * 1024 >> SECTOR_SHIFT)

#define DATA_DEV_BLOCK_SIZE_MAX_SECTORS (1024 * 1024 * 1024 >> SECTOR_SHIFT)

/*

 * FIXME: the cache is read/write for the time being.

static int parse_block_size(struct cache_args *ca, struct dm_arg_set *as,

			    char **error)

{

	unsigned long tmp;

	unsigned long block_size;

	if (!at_least_one_arg(as, error))

		return -EINVAL;

	if (kstrtoul(dm_shift_arg(as), 10, &tmp) || !tmp ||

	    tmp < DATA_DEV_BLOCK_SIZE_MIN_SECTORS ||

	    tmp & (DATA_DEV_BLOCK_SIZE_MIN_SECTORS - 1)) {

	if (kstrtoul(dm_shift_arg(as), 10, &block_size) || !block_size ||

	    block_size < DATA_DEV_BLOCK_SIZE_MIN_SECTORS ||

	    block_size > DATA_DEV_BLOCK_SIZE_MAX_SECTORS ||

	    block_size & (DATA_DEV_BLOCK_SIZE_MIN_SECTORS - 1)) {

		*error = "Invalid data block size";

		return -EINVAL;

	}

	if (tmp > ca->cache_sectors) {

	if (block_size > ca->cache_sectors) {

		*error = "Data block size is larger than the cache device";

		return -EINVAL;

	}

	ca->block_size = tmp;

	ca->block_size = block_size;

	return 0;

}